Sites sometimes use a bar in order to invite readers to subscribe to their mailing list.
In my experience, the majority of sites that do this choose a non-sticky top bar (header). However, it seems to me, intuitively, that this isn't a good choice, because readers are only exposed to it in the beginning, before they start engaging with the content.
This would obviously vary for different websites, but assuming that we're talking about regular blogs, I was wondering if:
There is generally a difference in performance between a top bar and a bottom bar (in terms of conversion rates). I'm interested both in data and in general best-practice guidelines.
There is a difference between the two in terms of when you should or should not use a sticky bar (i.e. it's okay to do this with a top bar, but not a bottom bar).